Fix typo in interface file

The get_lhs_offset function pointer type
in f32_bf16p_bf16p is fixed to read get_lhs_packed_offset
as the LHS is packed as well.

Signed-off-by: Felix Thomasmathibalan <felixjohnny.thomasmathibalan@arm.com>

Approved-by: Jakub Sujak <jakub.sujak@arm.com>